Recently, the effective utilization of water resources has been required due to the development of industry and increase in population. Thus, the reuse of waste liquid such as industrial wastewater is very important. In order to achieve the effective utilization and reuse, it is necessary to purify the waste liquid, namely to separate other substances from the waste liquid.
As a method of separating other substances from liquid, various methods have been known, and for example, membrane separation, centrifugal separation, active carbon adsorption, ozonation, aggregation, and further removal of suspended solids by a predetermined adsorbent, and so on have been exemplified. Such methods make it possible to remove chemical substances such as phosphorus and nitrogen contained in water, which greatly affect on the environment, and to remove oil, clay and so on dispersed in water.
Among them, the membrane separation is one of the most generally used methods, but has a problem that in the case of removing oil dispersed in water, the oil is likely to get caught in pores of a membrane so that the lifetime of the membrane is easily shortened. Thus, in many cases, the membrane separation is not appropriate for removing oil in water.
As a treatment method of such wastewater containing oil, there has been disclosed a method of treating hardly filterable waste liquid containing oil and so on with a filter auxiliary agent made of a magnetic substance, for example. Further, as a method of adsorbing oil in water, there has been cited a method of adsorbing oil with adsorbing polymers, for example, and thereafter removing the adsorbing polymers from water.